Acrolein is typically prepared in a partial gas phase oxidation of propene in the presence of a heterogeneous catalyst. The thus obtained acrolein containing product gas mixture also contains inert or non-condensable gases, such as nitrogen, oxygen, carbon monoxide, carbon dioxide, propylene or propane and by-products such as acetic acid, formaldehyde, allyl alcohol, and mainly acrylic acid. Subsequent treatments are therefore necessary to improve the purity of the crude acrolein. In a first treatment, the acrolein containing product gas mixture from the partial gas phase oxidation is fed to a quench column or quencher, where the heavy boilers, such as acetic acid and acrylic acid, are removed. A gas stream enriched in acrolein leaves the quencher. In the second treatment, the acrolein is separated from the gas stream leaving the quencher by absorption in water using an absorption column, at the bottom of which an aqueous acrolein solution is collected. In the third and final treatment, the aqueous acrolein solution is fed to a distillation column, where the low boiling acrolein is separated from the high boiling absorption medium and is recovered in liquid form.
A distillation is a separation unit, which utilizes vapor and liquid phases at essentially the same temperature and pressure for the coexisting zones. Within a distillation column means such as random or structured packings, and plates of trays are used to bring the two phases into intimate contact. Trays are stacked one above the other and enclosed in a cylindrical shell to form a column. Packings are also generally contained in a cylindrical shell between hold-down and support plates. A stream containing the compound to be purified is introduced into the distillation column at a single point between the top and the bottom of the distillation column, the so-called feed stage. This point also subdivides the distillation column into a rectifying section above the feed stage and a stripping section below the feed stage. The column may be operated continuously or in batch mode depending on a number of factors such as scale and flexibility of operations and solids content of feed. Because of the difference in density between vapor and liquid phases, liquid runs down inside the column, cascading from tray to tray, while vapor flows up the column, contacting liquid at each tray or within the packing. Liquid reaching the bottom of the column is partially vaporized in a heated reboiler to provide boil-up, which is sent back to the column. The remainder of the bottom liquid is withdrawn as bottoms, bottom stream or bottom product. Vapor reaching the top of the column is cooled and condensed to liquid in the overhead condenser. Part of this liquid is fed back to the column as reflux to provide liquid overflow. The remainder of the overhead stream is withdrawn as distillate, or overhead product. This overall flow pattern in a distillation column provides countercurrent contacting of vapor and liquid streams on all of the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibria to an extent dependent upon the efficiency of the contacting tray. The lighter components, i.e. the components with lower boiling temperatures, tend to concentrate in the vapor phase, while the heavier components, i.e. the components with higher boiling temperatures, concentrate in the liquid phase. The result is a vapor phase that becomes richer in lighter components as it passes up the column and a liquid phase that becomes richer in heavier components as it cascades downwards. The overall separation achieved between the distillate and the bottoms depends primarily on the relative volatilities of the components, the number of contacting trays in each column section, and the ratio of the liquid-phase flow rate to the vapor-phase flow rate in each section.
However, the person skilled in the art is faced with several big problems when purifying an aqueous acrolein solution in a standard distillation procedure. The first problem is that the condensation of the overhead vapor and its return to the top of the distillation column as reflux lead to a large holdup of the toxic compound acrolein in liquid and concentrated form within the distillation apparatus. Meaningful parameters for the evaluation of the toxicity of a chemical compound are amongst others the so-called ERPG (emergency response planning guidelines) values. ERPGs are exposure guidelines designed to anticipate health effects from exposure to certain airborne chemical concentrations. A chemical may have up to three ERPG values, each of which corresponds to a specific tier of health effects. The three ERPG tiers are defined as follows: a) ERPG-3 is the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects, b) ERPG-2 is the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protection action, and c) ERPG-1 is the maximum airborne concentration below which nearly all individuals could be exposed for up to 1 hour without experiencing more than mild, transient adverse health effects or without perceiving a clearly a clearly defined objectionable odor. Accordingly, the lower the ERPG values for a chemical compound, the higher is the risk potential of said chemical compound. With ERPG values of 0.05 ppm (ERPG-1), 0.15 ppm (ERPG-2) and 1.5 ppm (ERPG-3), acrolein is among the chemicals with the highest risk potential. Its storage therefore should be avoided and the holdup of acrolein in a plant also should be kept as low as possible. The second problem often faced in purifying an aqueous acrolein solution by distillation are polymerization and fouling. This fouling is due to the deposition of a solid phase resulting from the polymerization of compounds such as acrylic acid and allylacrylate, or even acrolein itself.
For minimizing the presence of liquid acrolein and fouling, the published patent application US 2005/0103616 A1 teaches a process for the purification of acrolein, in which an aqueous solution containing 6 wt.-% of acrolein is introduced into a distillation column equipped at its base with at least one boiler and at its top with at least one condenser. A liquid, water comprising mixture is withdrawn at the base of the distillation column as bottom stream, and an acrolein comprising gas mixture is withdrawn at the top of the distillation column as overhead vapor stream. This overhead vapor stream contains from 30 to 70 vol.-% of water. It is therefore necessary to subject said overhead vapor stream to a further process step in order to provide for purified acrolein. In this further process step the overhead vapor stream withdrawn from the distillation column is fed to a (partial) condenser, where the overhead vapor stream is cooled to a temperature, which makes it possible to obtain an aqueous condensate, which is fed back as reflux to the top of the distillation column, and an acrolein-rich gas mixture. However, the thus obtained aqueous condensate still contains ca. 20 wt.-% of acrolein. The additional use of the (partial) condenser, therefore, leads to a relatively large total holdup of acrolein, in particular liquid acrolein, in the distillation apparatus as a whole, and in particular in said condenser. However, in a given apparatus like a column or an external condenser connected to a column the total hold of a toxic compound, in particular in liquid form, should be kept as low as possible—a requirement which the process of US 2005/0103616 A1 does not meet. Further disadvantages of this process are the increased energy demand for operating the additional condenser and increased complexity of the used apparatus.
Hence, there was a need for an improved process for the purification of acrolein, which reduces the holdup of concentrated liquid acrolein in the distillation apparatus, in particular in the distillation column.